Beam current limiting circuit

ABSTRACT

A circuit for limiting the current supplied to the cathode of a grid controlled electron beam gun welder. When the current attempts to rise above a predetermined level because of a change of bias voltage, the voltage drop across a sensing resistor approaches that of a zener diode which clamps the base voltage of a transistor in the cathode circuit causing the transistor to shut off sufficiently to limit the current to the maximum set point level. An inductor in the cathode circuit also tends to limit beam current for a short duration change of bias voltage. Several circuits are connected in series.

"9 $1? I r g 7 19 /1,141 Q W ,1 M 7 .3 A, V "-3 ,1 1, 7,- 1113 331003279 M 1 5 Unlted States Patent 11 1 1111 3,760,279

Rudolph 1 Sept. 18, 1973 54] BEAM CURRENT LIMITING CIRCUIT 3,315,124 4/1967 Boeker 315/107 3,400,207 9/1968 Anderson 219/121 EB Inventor: Ralph 3,413,517 11/1968 Barber et a1. ..219 1211313 Borough, 2,544,340 3/1951 Maxwell 330/142 Assigneez United States steel C p Delagrange Pittsburgh, Pa.

Primary Examiner-John Korninski Flled: Sept. 16, Att0rney Rea C Helm 21 App1.No.: 181,131

[57] ABSTRACT [52] I g g ggl g; A circuit for limiting the current supplied to the cath- 51 I t Cl H02h ode of a grid controlled electron beam gun welder. {58} F'ge'ld EB 13] When the current attempts to rise above a predetermined level because of a change of bias voltage, the voltage drop across a sensing resistor approaches that of a zener diode which clamps the base voltage of a 328/259 168 transistor in the cathode circuit causing the transistor to shut off sufficiently to limit the current to the maxi- [56] References (med 4 mum set point level. An inductor in the cathode circuit UNITED STATES PATENTS also tends to limit beam current for a short duration 2,920,279 1/1960 Speller 330/138 change of bias voltage. Several circuits are connected 3,015,783 1/1962 Van Der Horst et al... 328/223 in erie 3,129,388 4/1964 Lang et al. 307/237 3,310,688 3/1967 Ditkofsky 307/237 3 Claims, 2 Drawing Figures I6 40 36 "w 1 M 34 42 HIGH VOLTAGE 9M5 POWER +5UPPLY SUPPLY 56 62 as T EL, 64

48 6'6 52 H/GH VOLT/16E 5 CONTROL 5a 5 F/LAMENT 30 l L;-,/-i SUPPLY T 38*... i

FIG. I.

"' BIAS +$UPPL Y F/LAME/VT /30 SUPPLY HIGH 0L TAGE CON TROL 800 F/LA MENT r0 GR/D HIGH VOLTAGE POWER SUPPLY BEAM CURRENT LIMITING CIRCUIT This invention relates to a beam current limiting circuit for use on a triode-type electron beam device and more particularly, to a current limiting circuit for use with a triode-type electron beam gun welder.

In triode-type electron beam devices, such as an electron beam welder, it is necessary to limit the electron beam current from exceeding a maximum value. With certain designs of high voltage power supplies used to power the electron beam gun, shifts in electron beam device parameters or electrical discharge of the high voltage from the electron beam device or related components can cause a sudden increase in electron beam current. Such an increase in electron beam current in the case of an electron beam welder is often detrimental to the welding process, increasing sufficiently to burn a hole through the welded material. The defect in the weld thus created may require lengthy repair procedures which are not always successful and creates increased material costs and operating time.

In accordance with my invention, a circuit is added to the cathode supply circuit which has a transistor in the cathode supply. A zener diode limits the biasing of the transistor and therefore the emitter-collector current flow. An inductor in the cathode supply further tendsto maintain a constant current for short duration system transients. Several circuits are connected in series to provide sufficient self bias voltage.

It is therefore an object of my invention to provide a circuit which prevents an electron beam current from exceeding a set point maximum.

Another object of my invention is to provide a circuit which tends to maintain a constant current for short duration system transients.

A further object is to provide a circuit which limits an electron beam current by increasing the negative bias on a control grid.

These and other objects will become more apparent after referring to the following specification and attached drawing in which:

FIG. I is a schematic drawing of an electron beam welder including the circuit of my invention; and

FIG. 2 is'a graph of the characteristics of an electron beam gun.

Referring more particularly to FIG. 1, reference numere] indicates a typical high voltage power supply providing between and 175 kilovolts at 170 milliamperes. High voltage power supply 10 has a positive terminal 12 connected to ground and a negative terminal 14 connected to a terminal A through a resistor 16. A cathode 18 of an electron beam gun 20 is connected to a terminal B through a variable resistor 22 and a fixed resistor 24. Electron beam gun 20 has an anode 26 connected to ground 28, the workpiece, and a filament supply 30. A bias supply 32 supplies a negative bias voltage through a resistor 34 and a resistor 36 to a grid 38 of electron beam gun 20. A diode 40 is connected between the junction of resistors 34 and 36 and terminal A. A resistor 42 is connected from the negative side of bias supply 32 to terminal A. A resistor 44 and a resistor 46 are connected in series between cathode l8 and ground. The junction between resistors 44 and 46 is connected to a high voltage control circuit 48 which has an output 50 controlling high voltage power supply 10 and an input reference 52. A corona discharge ring 54 is connected to grid 38. The parts thus far described is a conventional electron beam gun welder when terminals A and B are connected together.

The circuit of my invention, connected between terminals A and B, has a zener diode 56 and a resistor 58 connected in series between terminals A and B. An inductor 60 in parallel with a resistor 62 are connected in series with a variable resistor 64, the emitter of a transistor 66 and the collector of transistor 66 between terminals A and B. The base of transistor 66 is connected to the junction of zener diode 56 and resistor 58. A resistor 68 is connected between terminals A and B. Transistor 66 may be a high voltage Darlington pair, such as a 2N3439-2N3583, resistor 64 may be 25 ohms, resistors 58 and 68 each 15K ohms, resistor 62, ohms and inductor 60, 0.01 henries.

In normal operation, assuming that terminals A and B are connected thereby eliminating the circuit of my invention, high voltage supply 10 supplies a high voltage to gun 20 through resistor 16 and self-biasing resistors 22 and 24. Filament l8 emits the electron beam past grid 38 to anode 26 to the workpiece which is ground 28. Bias supply 32 supplies up to 3,000 volts negative bias to grid 38 through resistors 34 and 36. Diode 40 is not conducting for the direction of bias voltage shown. When electron beam gun 20 is producing a current, the current flow through resistors 22 and 24 provides a voltage drop which adds additional bias between filament 18 and grid 38.

The majority of electrical discharges occur from corona ring 54 to ground. When such a discharge occurs, the path of the discharge current is from high voltage power supply 10, through resistor 16, diode 40 and bias supply 32 in parallel, through resistor 36 to ground. The discharge current, driven by the high voltage, shorts out the output of bias supply 32 such that essentially there is no voltage drop across diode 40.

Referring to FIG. 2, a typical operating point is shown as location P-ll, milliamperes operating at kilovolts with a bias of about 800 volts negative with respect to filament 38. The setting of resistors 22 and 24 provides a self bias of about 375 volts of the 800 volt bias, the balance is provided by bias supply 32. When a discharge occurs, the 425 volts of the bias supply is lost and the beam current jumps to a value determined by the self-biasing limiting, which is location P-2, 216 milliamperes at 540 volts self bias. This is a power increase of 216/150 or 1.44, which is highly undesirable.

When terminals A and B are disconnected and the circuit of my invention connected as shown between terminals A and B, normal operating currents from about 5 milliamperes up to about milliamperes will supply a current through resistor 58 to the base of transistor 66 turning transistor 66 on such that no voltage drop occurs across it. Assuming an operating current of 150 milliamperes, and resistor 64 set for 25 ohms, there is a voltage drop of about 4.9 volts from the base of transistor 66 to terminal A, 3.9 volts between terminal A and the emitter of transistor 66 and about 1 volt between the emitter and base of transistor 66. At this voltage zener diode 56 will not conduct, but the characteristics of zener diode 56 are such that when the voltage between terminal A and the base of transistor 66 reaches 6 volts, zener diode 56 will conduct.

When a discharge occurs, the operating point will move from P-l towards 1P-2 as shown in FIG. 2. However, when the current reaches 190 milliamperes, the

voltage between terminal A and the base of transistor 66 will be 6 volts, the level at which zener diode 56 will conduct. For currents beyond 190 milliamperes, zener diode 56 will then clamp the voltage between terminal A and the base of transistor 66 at 6 volts, and the voltage between the base and emitter will drop below 1 volt as the higher current tends to raise the voltage drop between terminal A and the emitter of transistor 66. Since transistor 66 has a very large beta, about 10,000, the drop in voltage between base and emitter tends to turn the transistor off sufficiently to prevent a current increase beyond about 190 milliamperes by increasing the effective resistance and thereby increasing the voltage between the emitter and collector of transistor 66. The increased resistance tends to create additional self bias voltage to limit the beam current providing a static current limiting.

Since each transistor has a design characteristic enabling it to handle up to 300 volts between the emitter and collector, several circuits are connected in series between terminals A and B. Thus five circuits in series could handle about 1,500 volts of self bias, more than sufficient to hold the beam current at 190 milliamperes.

When a short duration discharge occurs, inductor 60 acts as a high resistance, decaying exponentially, providing dynamic current limiting. For a pulse disharge the voltage between terminal A and the emitter of transistor 66 reaches volts at whatever operating current is used holding that current for the duration of the short discharge. The longer the discharge, the less the dynamic blocking occurs up the static limit of 190 milliamperes.

Zener diode 56 and resistor 64 can be selected for almost any current limit desired. Inductor 60 and resistor 62 may also be selected to provide a desired dynamic response. Resistor 64 may be varied for any particular maximum current setting desired. Equalizing resistor 68 is desirable, especially if several circuits are used in series, but not absolutely necessary if the circuits are well matched. Resistor 68 provides a minimum resistance for each circuit so that careful matching of individual circuit components is not necessary to prevent exceeding maximum transistor voltage ratings.

I claim:

1. In an electron beam device having a high voltage supply, a bias voltage supply connected between the grid and the high voltage supply, a self-biasing resistor connected in the cathode circuit, and an operating characteristic whereby a decrease in bias voltage results in an increase in the electron beam current, a circuit for providing bias when transients cause loss of the bias voltage supply comprising a transistor in the cathode circuit having a collector connected to the cathode, a zener diode connected between the high voltage supply and the base of the transistor, an impedance connected between the high voltage supply and the emitter of the transistor, the value of said impedance being selected to provide a voltage drop across the impedance within a range of normal operating currents which is much less than the conduction voltage of the zener diode, and a first resistor connected between the base and the collector of the transistor.

.2. A circuit according to claim 1 in which the value of the first resistor is selected so that the transistor is fully on within a range of normal operating currents.

3. A circuit according to claim 1 in which said impedance includes a second resistor, an inductor connected in parallel with the second resistor and a third resistor connected in series with the second resistor and the inductor, said second resistor, said third resistor and said inductor each having values selected whereby a tendency for a sudden increase in operating current will tend to turn the transistor off.

i t t t 

1. In an electron beam device having a high voltage supply, a bias voltage supply connected between the grid and the high voltage supply, a self-biasing resistor connected in the cathode circuit, and an operating characteristic whereby a decrease in bias voltage results in an increase in the electron beam current, a circuit for providing bias when transients cause loss of the bias voltage supply comprising a transistor in the cathode circuit having a collector connected to the cathode, a zener diode connected between the high voltage supply and the base of the transistor, an impedance connected between the high voltage supply and the emitter of the transistor, the value of said impedance being selected to provide a voltage drop across the impedance within a range of normal operating currents which is much less than the conduction voltage of the zener diode, and a first resistor connected between the base and the collector of the transistor.
 2. A circuit according to claim 1 in which the value of the first resistor is selected so that the transistor is fully on within a range of normal operating curreNts.
 3. A circuit according to claim 1 in which said impedance includes a second resistor, an inductor connected in parallel with the second resistor and a third resistor connected in series with the second resistor and the inductor, said second resistor, said third resistor and said inductor each having values selected whereby a tendency for a sudden increase in operating current will tend to turn the transistor off. 